Answer

To flatten an array to any nesting depth, recursively traverse nested arrays until no nested arrays remain.

function flattenDeep(array) {
  const result = [];

  for (let i = 0; i < array.length; i++) {
    if (!(i in array)) {
      continue;
    }

    const value = array[i];

    if (Array.isArray(value)) {
      result.push(...flattenDeep(value));
    } else {
      result.push(value);
    }
  }

  return result;
}

console.log(flattenDeep([1, [2, [3, [4, 5]]]]));
// [1, 2, 3, 4, 5]

Explanation

This solution uses recursion and the Array.isArray() check. Each nested array is flattened independently, and its result is appended to the parent result.

The spread operator can hit argument-count limits for extremely large arrays. For production code handling large inputs, append elements iteratively instead.